The present invention relates to a method and apparatus for remotely monitoring the temperature of a body. The invention is particularly useful for remotely monitoring the temperature of a semiconductor wafers and is therefore described below with respect to this application.
In the microelectronics and optoelectronics industries, semiconductor wafers and other devices are subjected to various multistage processes, including plasma etching, chemical vapor deposition, plasma vapor deposition, ion implantation, molecular beam epitaxy, etc. During the processing of such semiconductor devices, it is critical to monitor the temperature of the device at each stage, not only to achieve the required properties of the layer in the device being created or modified in the respective stage, but also to ensure that the temperature does not exceed that which will cause degradation of the layers created or modified in preceding stages. At the present time, the technique generally used for remotely sensing the temperature of a semiconductor wafer during processing is radiometric and relies on sensing the intensity of infrared black body radiation emitted by the semiconductor.
Arima et al., U.S. Pat. No. 4,979,134, describes a method for measuring surface temperature of a semiconductor wafer substrate; however, the method therein described is limited to wavelengths less than 1 micron, where the semiconductor is opaque and is therefore limited to temperatures above about 500.degree. K.
Walter, U.S. Pat. No. 4,841,150, describes a method and apparatus for thermal mapping of semiconductors by a reflectivity technique which relies on a shift with temperature in the wavelength of light corresponding to the band gap energy. However, the technique described in that patent cannot be used as a practical manner to determine the temperature of real life process semiconductor wafers, because in the vast majority of cases, the presence of back and front side films causes constructive and destructive optical interference phenomena which swamp the band edge optical phenomenon.
Another technique is described by Kirillov et al. in U.S. Pat. No. 5,118,200. Here the substrate is illuminated with a source of continuous spectrum light and the exiting light is analyzed to determine its intensity as a function of wavelength. The wavelength at the point of inflection in the wavelength-intensity curve is used to is determine the substrate temperature. Although the point of inflection technique reduces the need for precise information of the spectral location of the energy gap in the temperature measurement procedure, it does not compensate for deviations in the spectral position of the point of inflection due to the presence of thin dielectric or semiconductor films on the wafer as expected in a real life processing environment.
In addition, neither the Walter nor the Kirillov patents describes embodiments which are capable of measuring the temperatures of wafers which are opaque due to highly doped bulk or due to the presence of opaque films.
The use of reflection intensity information in conjunction with Kirchoff's Law in order to compensate for unknown variations in emissivity is well known and has been an important aspect in the literature for general temperature measurement, for example U.S. Pat. No. 3,433,052 of Maley et al.
Nulman et al., U.S. Pat. No. 4,919,542 have utilized this concept for the case of temperature measurement of semiconductors in the wavelength band of from 4.5 to 6 microns. However, the technique described there is limited for two reasons. First, the technique is only applicable to wafers which are opaque in this wavelength regime (i.e., above 600.degree. K or highly doped), unless complex calibration procedures employing contact thermometry are used. Second, the required precision of emissivity compensation is very high because of the low radiance contrast at these long wavelengths.
Crowley et al., U.S. Pat. No. 4,969,748, rely on a similar reflectivity-based emissivity compensation technique at a single undisclosed wavelength in a separate chamber prior to processing. A detailed description is given of the algorithm used to compare data taken on sample wafers and compensate for the variations with temperature.
Moslehi et al., U.S. Pat. No. 5,156,461, also relies on reflection properties of the semiconductor in order to compensate for emissivity. In this case the optical setup allows for multipoint measurement of emissivity and hence temperature.
None of these references offers a satisfactory solution for the case of transparent wafers at low temperatures. Furthermore, none makes any explicit reference to the use of more than a single wavelength by the pyrometer, and none has the capability of varying the pyrometer wavelength during the temperature measurement.
Gat et al., U.S. Pat. No. 5,165,796, describe a temperature measurement apparatus which relies on self-emission measurements from a semiconductor wafer in two or more wavelength bands, 3.4 and 4.7 microns in the preferred embodiments. These measurements are carried out simultaneously. However, the exact algorithm utilizing the dual or multiple wavelength information is not explicitly described. As with the techniques previously described, this technique is also not applicable to transparent wafers at low temperatures.
Finally, Stein, U.S. Pat. No. 4,708,493, utilizes dual wavelength emission and reflection measurement to compensate for emissivity variations but continues to rely on target opacity.
It is therefore apparent that there remains a need to determine by non-contact means the temperature of real life wafers in a process environment.
One way of remotely monitoring the temperature of a body includes projecting light of known spectral content onto a surface of the body, detecting the light reflected from the body, and measuring the shift in the spectral location of the optical absorption edge in the reflected light as compared to the projected light, to thereby provide a measurement of the temperature of the body.
The body may be a light-transmissive body, particularly a semiconductive body, in which the light detected is that reflected directly from the near side of the body, as well as that passing through the body and reflected back through the body from the far side.
Such a method is particularly useful for remotely monitoring the temperature of a semiconductor wafer as it is being processed, and provides a number of important advantages over the existing temperature measuring techniques. Thus, the temperature range over which the technique is applicable to semiconductor wafers is from 900.degree. K down to absolute 0.degree. K, in contrast with the radiometric technique which is virtually inapplicable at wafer temperatures below 500.degree. K. The processing of the semiconductor wafer consists of depositing thin layers of materials such as metals on one surface of the wafer. The light used to monitor the temperature of the wafer is directed at the opposite surface of the wafer. The surface whereon the thin layers are deposited is conventionally called the "front" surface of the wafer, and the opposite surface is conventionally called the "back" surface of the wafer. The same convention is followed herein: the surface from which the incident light is reflected directly is called herein the "back" surface, and the surface from which the incident light is reflected after passing through the body is called herein the "front" surface, it being understood that these designations are arbitrary, and do not limit the scope of the present invention to directing light at the back surface of a semiconductor wafer.